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[nelsona]

I recently saw on Youtube a piston ballast system just pumping water into the cylinder and sending the piston to the far end thus sinking the boat, On reversing the pump the water is of course removed, a sort of hybrid of the piston and motor system, any observations? It does seem to work

[Sub driver]

they are called piston tanks very reliable and very common. check out the engel modelbau website

[nelsona]

Hi Subdriver, I was aware of the mechanical (motor and screwed rod) method, but if you take a look at Youtube there is a piston that appears to just have water pumped into the cylinder (via a tube) withdrawn by reversing the pump i.e. no screwed rod or mechanical connection as far as I can see!

[Mankster]

Thats nothing new, pretty much the same as pumping water in and out of say a saline bag. Yes it works just fine.

[Martin (Admin)]

           http://www.engel-modellbau.eu/catalog/product_info.php?cPath=2_29&products_id=1736

[Davy1]

Yes that is a screw driven piston tank.

I have heard of the floating piston tank arrangements.

As Mankster says, I can't think of any improvement over the use of bags. May be more resistant to overpressure and bursting?

Certainly not as  precise as a screw driven one.

David

[Albion]

sounds like the RPM tech system

http://www.rpmtech1.com/

was it one of these?

[Subculture]

Piranha Modellbau did something similar too. They used a peristaltic pump with a syringe to convert a Seawolf to a static diver.

One advantage of using a piston over a bag is that you can apply feedback to its position. Therefore it should be possible to make it work like a servo controlled tank, only you won't need a lathe to make a spindle nut etc.

Another idea would be to put an encoder on the wheel of a peristaltic pump, then you could just count the revolutions. Don't know if anyone has tried it, haven't seen it done up until now.